1. The Field of the Invention
The present invention is in the field of hydrocarbon fuels, more particularly in the field of processing hydrocarbons, such as diesel and biodiesel, in order to increase the cetane number.
2. The Relevant Technology
Cetane number is a measurement of the combustion quality of diesel fuel during compression ignition. It is a significant expression of diesel fuel quality among a number of other measurements that determine overall diesel fuel quality. Cetane number is actually a measure of a fuel's ignition delay, which is the time period between the start of injection and start of combustion (ignition) of the fuel. For any given diesel engine, a higher cetane fuel will have a shorter ignition delay period than a lower cetane fuel.
Generally, diesel engines run well with a cetane number from 40 to 55. Fuels with higher cetane numbers and shorter ignition delays provide more time for the fuel combustion process to be completed. This, in turn, increases the extent and efficiency of combustion. Higher speed diesel engines operate more effectively when using higher cetane number fuels. Nevertheless, there is typically no performance or emission advantage when the cetane number is increased beyond approximately 55. Beyond this point, the fuel's performance hits a plateau.
By way of background, cetane is an un-branched, open chain, alkane molecule that ignites very easily under compression, so it was assigned a cetane number of 100. Conversely, alpha-methyl napthalene was assigned a cetane number of 0. All other hydrocarbons in diesel fuel are indexed to cetane as to how well they ignite under compression. The cetane number therefore measures how quickly the fuel starts to burn (auto-ignites) under diesel engine conditions (i.e., compression and temperature). Since there are hundreds of components in diesel fuel, with each having a different cetane quality, the overall cetane number of the diesel is the average cetane quality of all the components. There is typically very little actual cetane in diesel fuel.
In North America, most states adopt ASTM D975 as their diesel fuel standard, and the minimum cetane number is set at 40, with typical values in the 42-45 range. Premium diesel fuels may or may not have higher cetane numbers, which is supplier dependent. Premium diesel fuels often include additives to improve cetane number and lubricity, detergents to clean the fuel injectors and minimize carbon deposits, water dispersants, and other additives depending on geographical and seasonal needs.
In Europe, diesel cetane numbers were set at a minimum of 38 in 1994 and 40 in 2000. The current standard for diesel sold in Europe is determined by EN 590, with a minimum cetane index of 46 and a minimum cetane number of 51. Premium diesel fuel can have a cetane number as high as 60 in Europe.
Additives such as alkyl nitrates (e.g., 2-ethyl hexyl nitrate), di-tert-butyl peroxide, and dimethyl ether are commonly used as additives to raise the cetane number. Additives such as 2-ethyl hexyl nitrate are very expensive, costing approximately $2200/ton, and cannot be used in quantities greater than about 0.2% of the diesel fuel without becoming cost prohibitive.
Biodiesel from vegetable oil sources have been recorded as having a cetane number range of 46 to 52. Animal-fat based biodiesels cetane numbers range from 56 to 60.
The cetane number of diesel fuel can also be increased by processing diesel fuel having a lower cetane number to yield a diesel fuel having a higher cetane number. For example, U.S. Pat. No. 5,114,434 to Praulus et al. describes a process by which viscoreduced diesel fuel is contacted with hydrogen peroxide in a reactor that includes a stirring mechanism. While the process disclosed by Praulus et al. effectively increased the cetane number, the amount of increase was modest (i.e., the cetane number was increased from 39 to 50 in one example and from 39 to 53.5 in another). Moreover, the residence time in the reactor was quite long, being 5 hours or more.
Other processes are designed to primarily desulfurize high sulfur fossil fuels by maximizing oxidation of sulfur-bearing molecules and formation of water-soluble sulfones that can be removed by phase separation. An example of such processes is described in U.S. Pat. No. 6,500,219 to Gunnerman. In the case of high sulfur diesel, Gunnerman provide reaction conditions (e.g., preheating of the diesel, coupled with no cooling of the reaction) that promote oxidation of sulfur-bearing molecules, while minimizing oxidation of other hydrocarbon molecules.